Conventionally, metal sheets formed of tin-free steel (TFS), aluminum, or the like, which are metal can materials used for food cans, have been painted to improve corrosion resistance, durability, weather resistance, and the like. However, the painting technique has problems of a complex baking process, a long treatment time, and a large amount of solvent discharged.
To solve these problems, such painted metal sheets have been replaced by resin-coated metal sheets obtained by laminating a thermoplastic resin film on a heated metal sheet. Currently, resin-coated metal sheets are widely used mainly as materials for beverage cans in the industrial field.
However, the use of resin-coated metal sheets in food can applications causes a problem in that it is difficult to take contents out of a container because the contents strongly adhere to the inner surface of the container when the contents are taken out of the container. Since this problem closely relates to consumer purchasing intention, it is very important to solve this problem in order to increase consumer purchasing intention. However, there are very few studies on an improvement in content release property for conventional resin-coated metal sheets.
The inventors of the present invention have diligently carried out studies to obtain a content release property. As a result, the inventors have found that a good content release property for contents containing a large amount of fatty components (e.g., mixtures of meats, eggs, and carbohydrates) can be obtained by adding a particular wax to a polyester resin such that the particular wax is present on the surface of the resin. Therefore, the inventors have filed Patent Literature 1.
The inventors have filed Patent Literatures 2 and 3 as a result of subsequent studies in which improvements have also been made in order to obtain a good content release property for contents having a high protein content (such as luncheon meat) and strong adhesiveness.